This invention relates to an aqueous synthetic dispersion based on a copolymer of acrylic monomers and/or vinyl monomers where the copolymer contains at least two different monomers that are crosslinkable with each other, as well as emulsifiers.
Aqueous copolymer dispersions are of particular interest for the production of thermosetting coatings because they do not burn and as they are physiologically less precarious than solvent containing coatings. Moreover, recovery of expensive solvent can be dispensed with. Besides the properties demanded of all synthetic dispersions such as good flow, good processability, absence of coagulate, storage stability and temperature resistance, it is also particularly important for dispersions which shall be used as binders for the production of thermosetting coatings that glossy coating materials with good leveling characteristics and good adhesion are obtained which, after thermal treatment, are largely resistant to solvents, particularly to ketones, alcohol and hydrocarbon mixtures. After thermal treatment, those coating materials shall be hard, scratch resistant and, under the influence of heat, blocking resistant. To this end the copolymers forming the basis of the dispersions contain at least two chemically different groups which are introduced into the molecule by polymerization and which, after the film has dried up, lead to crosslinking of the previously thermoplastic copolymer during heating.
It is generally known, as shown in DE-OS 31 47 007, to produce thermosetting coating materials, containing as binders, aqueous dispersions based on acrylic monomers by copolymerization of N-methylolacrylamide and hydroxyalkyl(meth)acrylates. However, during curing those dispersions give off larger amounts of formaldehyde which have to be intercepted with urea so that the water resistance of the corresponding products is reduced.
It is also known to produce solvent containing acrylic resins containing hydroxyl groups and alkoxymethyl(meth)acrylamide groups for crosslinking. However, those systems contain solvents and, because of the low molecular weight of the polymer resin, they will not reach sufficiently high molecular weight, even not with crosslinking, such that the gloss is retained under the influence of solvent or weathering.
Thus, it is an object of the present invention to produce an aqueous dispersion of a copolymer which is selfcrosslinking when heated and results in glossy, solventresistant coatings with good adhesion to metallic and plastic substrates.